The invention relates to block copolymer blends having a superior combination of physical properties, particularly improved resistance to oil absorption and improved tensile strength. More particularly, the invention is directed to blends of hydrogenated block copolymers having few side chains on the elastomeric midblocks and block copolymers having higher amounts of side chains on the midblocks.
The block copolymers usable within the scope of the invention include those having a configuration A-B-A. Block copolymers of the A-B-A configuration are known, see U.S. Patent No. Reissue 27,145, which discloses, among other polymers, block copolymers having an A-B-A configuration wherein the two terminal blocks A comprise thermoplastic polymer blocks of a vinyl arene such as polystyrene(s), and the mid-block, block B, is a polymer block of a selectively hydrogenated conjugated diene such as hydrogenated polybutadiene (EB) or hydrogenated polyisoprene (EP). The proportion of the thermoplastic terminal blocks to the center elastomeric polymer block, the relative molecular weight of each of these blocks, and the number of side chains on the B blocks is balanced to obtain a rubber having a combination of properties, such that it behaves as a vulcanized rubber without requiring the actual step of vulcanization.
Hydrogenation may be effected selectively on the B blocks when desired, as disclosed in U.S. Patent No. Reissue 27,145. The '145 patent describes a block copolymer wherein each A block is a polymerized monoalkenyl aromatic hydrocarbon block having an average molecular weight of about 4,000 to 115,000; and each B block is a polymerized butadiene hydrocarbon block having an average molecular weight of about 20,000 to 450,000. Further, each A block constitutes from 2 to 33 weight percent of the copolymer, and each B block preferably contains from 35 to to 55 mole percent of condensed butadiene units in a 1,2 configuration, a measured by a standard NMR technique. Finally, the unsaturation of the B blocks is preferably reduced to less than 10% of the unsaturation of the non-hydrogenated polymer.
While hydrogenated block copolymers having from 35-55% of the polybutadiene in the 1,2-configuration have a number of outstanding technical advantages as described in Reissue 27,145, one of their principal limitations lies in their ability to absorb oil which can significantly reduce tensile strength. The ability of the block copolymer to absorb oil is essentially due to its non-crystalline hydrocarbon character which is not materially changed by hydrogenating the copolymer when greater than 35% of the polybutadiene is in the 1,2 configuration.
The Reissue Patent No. 27,145 also teaches the composition and tensile properties of block copolymer having less than 35% of the polybutadiene in the 1,2 configuration. However, these compositions were considered to be too stiff probably due to crystallization of the B blocks.